(a) Field of the Invention
The present invention relates to a field emission display, and more particularly, to a field emission display having emitters made of carbon nanotubes.
(b) Description of the Related Art
The field emission display (FED) uses a cold cathode as the source for emitting electrons to realize images. The overall quality of the FED depends on the characteristics of emitters, which form an electron emitting layer. The first FEDs utilized emitters made mainly of molybdenum (Mo), that is, the emitters were formed of what are referred to as Spindt-type metal tips. As an example of such prior art technology, there is disclosed a display system that has a field emission cathode in U.S. Pat. No. 3,789,471.
However, during manufacture of the FED having metal tip emitters, since a semiconductor manufacturing process is used, which includes photolithography and etching processes to form holes in which emitters are provided and the process of depositing molybdenum to form metal tips, not only is production complicated and a high technology is needed, but expensive equipment is required, thereby increasing overall unit costs. These factors make mass production of such FEDs problematic.
Accordingly, much research and development is being performed by those in the field emission display industry to form emitters in a flat configuration that enable electron emission at low voltages (10˜50V) and a simple manufacture of the emitter structure. It is known that carbon-based materials, for example, graphite, diamond, DLC (diamond-like carbon), C60 (Fullerene), or carbon nanotubes, are suitable for use in the manufacture of planar emitters. In particular, it is believed that carbon nanotubes, with their ability to realize electron emission at relatively low driving voltages of approximately 10˜50V, is the ideal emitter configuration for FEDs.
U.S. Pat. Nos. 6,062,931 and 6,097,138 disclose cold cathode field emission displays that are related to this area of FEDs using carbon nanotube technology. The FEDs disclosed in these patents employ a triode structure having cathodes, an anode, and gate electrodes. During manufacture of these FEDs, cathode electrodes are first formed on a substrate, then after providing emitters on the cathode electrodes, the gate electrodes are formed on the emitters. That is, the prior art FEDs have a structure in which the gate electrodes are provided between the cathode electrodes and an anode electrode, and electrons emitted from the emitters are induced toward a phosphor layer.
To improve the characteristics of the FED, the above triode structure is used and the emitters are formed using a carbon-based material, that is, carbon nanotubes. However, it is difficult to precisely form the emitters in holes formed in an insulation layer, which is provided under the gate electrodes. This is a result of the difficulties involved in forming the emitters with a printing process that uses paste. In particular, it is very difficult to provide the paste in the minute holes for formation of the emitters.
Further, with respect to the FED having the conventional triode structure, when the electrons emitted from the emitters form electron beams and travel in this state toward their intended phosphors, there are instances when an excessive diverging force of the electron beams is given by gate electrodes when passing a region of the gate electrodes to which a positive voltage is applied. In such a case, the electron beam emitted from an emitter illuminates a phosphor adjacent to the intended phosphor as a result of the undesirable spreading of the electron beams. Therefore, color purity and overall picture quality deteriorate.
To remedy this problem, there has been disclosed a configuration in which a metal grid of mesh type is provided between the cathode electrodes and anode electrode in an effort to realize good focusing control of the electrons emitted from the emitters. Japanese Laid-Open Patent No. 2000-268704 discloses such an FED.
In an FED having the metal grid, in addition to the advantages described, arcing results from the high voltage applied to the anode electrode such that damage to the cathode structure, which includes the emitters, is prevented. However, when electron beams are emitted from the emitters, the electron beams are unable to pass through holes formed in the metal grid and instead strike the metal grid to thereby decrease the utilization efficiency of the electron beams. Hence, because the final number of electron beams reaching the phosphors is lower than needed, brightness of the picture is reduced.
Such a problem may become worse in FEDs in which the gate electrodes are provided under the cathode electrodes on a substrate and the emitters are formed on the cathode electrodes (e.g., U.S. Pat. No. 6,420,726 disclosed by the assignee). This is because most of the emission of the electron beams occurs in the edges of the emitters. If the electron beams do not pass through the metal grid unimpaired, the number of electron beams for illuminating the phosphors is significantly reduced.
In all display devices including the FED, the light emitting source (cold cathode electron emission in the case of FEDs) must uniformly illuminate the pixels to provide for good picture quality. However, the above structure of the emitters in which the emitters are arranged in edge portions of the cathode electrodes is unfavorable for uniformly emitting electrons to each pixel.
This is a result of the small contact area between the emitters and the cathode electrodes that causes an increase in the contact resistance that interferes with electron emission. Further, when the emitters are formed on the cathode electrodes, the arrangement of the emitters is not uniform so that electron emission occurs in sections.